Electron Microscopy Lab Adds New Instrument

From nanoscience and quantum computers to the human genome, the subjects of scientific research keep shrinking. Whether studying quantum dots and rods or bacterial toxin proteins, researchers must now understand and manipulate interactions that take place at the atomic scaleÑgenerally measured in a few nanometers, or billionths of a meter.

The Electron Microscopy Lab (EML) recently increased researchersÕ ability to probe such phenomena by adding a new analytical scope to its fleet of electron microscopes. The $2.4 million FEI Tecnai F30 is a transmission and scanning-transmission electron microscope that allows researchers to see the structure of materials at very nearly the atomic scale. ÒIn much of science, seeing is believing,Ó said EMLÕs Rob Dickerson, Òand this is where you can do your seeing.Ó

Electron microscopes are similar to light microscopes but focus a beam of electrons with magnetic lenses rather than refracting light through optical lenses. Also, since the electrons would be scattered by collisions with air molecules, imaging must take place in a vacuum. And given the high energy of electron beams, specimens are viewed indirectlyÑon a fluorescent screen, a charge-coupled device camera, or photographic film.

It is the short wavelength of electron beams that gives electron microscopes their high resolution. Because this wavelength depends on the voltage that accelerates the electrons into a specimen, accelerating voltages of 100 kilovolts or more are used. At these voltages, electrons have wavelengths of about 0.001 to 0.004 nanometer, yielding resolutions on the order of 0.1 to 0.2 nanometer, 1,000 times better than those of the best light microscope. Such resolutions can image viruses and large biological molecules and probe the atomic microstructures of materials. By contrast, the shortest wavelength of visible light yields, at best, a resolution of 0.2 micrometer, just good enough to image bacteria.

The new Tecnai can function either as a transmission electron microscope (TEM) or as a scanning-transmission electron microscope (STEM). TEMs function like an upside-down light microscope, with electrons passing through the specimen. Image contrast is formed by elastic and inelastic scattering of the electrons within the specimen. The Tecnai has a directly interpretable Òpoint-to-pointÓ resolution of 0.21 nanometer in the TEM mode.

By contrast, in the TecnaiÕs STEM mode, a finely focused beam of electrons is scanned over the specimen, and an image is formed by using the transmitted beam, diffracted beams, or higher-angle inelastically scattered electrons. Images and chemical information with a resolution of 0.35 nanometer can be obtained by using the STEM mode together with detectors that analyze emitted x-rays or energy loss in the transmitted electron beam. Taken together, these capabilities make for a highly flexible microscope, thus meeting the needs of the weapons program, which provided the capital funds to purchase the instrument. One program application is to characterize plutonium alloys.

The EML is a user facility available to researchers both inside and outside the Laboratory. Current or recent EML research projects include studies of beryllium-containing aerosol samples that could relate to lung pathology in chronic beryllium disease, the structures found in uranium carbides, and the deformation microstructures found in nanolayered composite materials. The EML is also used by small businesses and is available as a forensic tool to state agencies.
ÑKevin N. Roark and Vin LoPresti